The present invention relates to a safety switching module for safely switching-off an electrical load, comprising a first and a second switching control device, a first and a second switching element, both being series-connected with each other and forming a first current path for supplying the load, whereby the first switching element can be controlled by the first switching control device and the second switching element can be controlled by the second switching control device, and an evaluation and control device for testing the switching-off ability of at least one switching element. The present invention further relates to a method for testing the switching-off ability of a switching element in such a safety switching module.
Such safety switching modules and safety switching devices, respectively, are generally known. For example, the applicant offers safety switching devices of different types under the name “PNOZ”. In the patent application DE 100 11 211, the applicant has disclosed such a safety switching device, for example.
In general, such safety switching devices are particularly used in the industrial field as to switch on and safely switch off electrically driven machines, like a stamping press or a milling cutter, a valve block for pneumatic or hydraulic control systems or output models of an SPS. Particularly in connection with a mechanically-operated emergency stop push button they serve to switch off the machine in an emergency situation quickly and safely. For this purpose, the energy supply of the machine to be switched off is coupled via two switching elements connected in series. As soon as just one of both switching elements opens, the energy supply to the machine is interrupted.
In order to carry out a test of the switching ability of the switching element during operation of such a safety switching device, an evaluation and control unit is provided which switches off each single switching element briefly and which thereby detects and evaluates the output signal (back-read-out signal) of the switching elements. On the basis of this evaluation, the evaluation and control unit is able to determine whether each switching element has the ability to interrupt the electrical supply of the machine, i.e. in general the electrical load, in an emergency situation. In order not to effect the electrical load during this test procedure, the switching element is switched off just for a very brief time period which is not “viewable” for the load.
Due to the mechanical inertia of the electromechanical switching elements, this test procedure is possible only for electronic switching elements on the basis of semiconductor devices. If, however, the current to be switched by the switching element exceeds a predetermined value (typically about 8 ampere) and the electrical load is not a pure ohmic load (capacitive and inductive components), the above-mentioned test procedure consisting of the brief switching-off of the switching elements is only possible with a large additional effort. For evaluating the output signals of the switching elements, the load would have to be discharged in this case as to keep the switching-off pulse small. The brief discharge of a larger capacitor would require a very large discharge current. For example, to discharge a 1 mF capacitor in a time period of 200 μs by 25 V, a discharge current of 125 A would be necessary.